1. Field of the Invention
This invention relates to spectrally sensitized silver halide photographic emulsions and, more particularly, to those supersensitized with combinations of sensitizing dyes. Still, more particularly, the invention relates to emulsions having a markedly increased spectral sensitivity in the green region of the spectrum.
2. Description of the Prior Art
In the technical field of the production of photographic materials, it is well known to spectrally sensitize emulsions in order to extend the spectral response to a longer wavelength. In these sensitization techniques, sensitization at the green region is especially important partly because the maximum sensitivity of human vision lies at about 545 nm with the human eye being most sensitive to light in the green region.
In the field of radiography, an intensifying screen or a fluorescent panel is frequently employed in combination with a silver halide photographic film in order to increase the recording sensitivity. Many attempts have been made to improve the recording speed for X-ray radiation, which will result in the prevention of harmful effects to the human body accompanied by an excessive X-ray dosage, an improved detection of fine details with a smaller dosage, and also in an extension of the X-ray recording range. Such attempts include, for example, development of techniques to improve the sensitivity of silver halide photographic emulsions, development of systems employing X-ray image intensifiers and development of systems employing solid-state X-ray amplifiers. It should be noted, however, that in all of these systems the final process is to record a fluorescent light image onto a silver halide photographic material.
Fluorescent materials used for the present purpose include blue light emitting materials such as barium sulfate activated with strontium, barium sulfate activated with lead, barium sulfate activated with silver, calcium tungstate activated with lead, zinc sulfide activated with silver, and barium phosphate (Ba.sub.3 (PO.sub.4).sub.2) activated with europium, and green light emitting materials such as zinc-cadmium sulfide activated with silver.
Recently oxy-sulfides of yttrium or of the lantanide elements activated with rare earth elements such as, for example, gadolinium oxy-sulfide activated with terbium (Gd.sub.2 O.sub.2 S) are known to fluoresce with a high emitting energy intensity as disclosed in Lockheed Aircraft Corporation research reports. Further, as other members of this family, lanthanum oxy-sulfide, yttrium oxysulfide, etc. are also described in U.S. Pat. Nos. 3,721,827, and 3,705,858.
On the other hand, the photo-sensitive x-ray recording materials used including direct and indirect x-ray recording materials should preferably be handled including developing and fixing operation under illumination conditions as bright as possible. As a matter of fact these x-ray recording photographic materials are processed and handled under a safe light provided with a No. 7 filter produced by Fuji Photo Film Co., which has the spectral transmitting curve shown in FIG. 1.
As is evident from the above descriptions, X-ray recording materials based on silver halide photographic emulsion must be highly sensitive to the light emitted by the x-ray excitation and at the same time weakly sensitive to the light used as a safe light.
With the arrival of the information age, an urgent need arose for the development of information transmitting systems with higher transmitting speeds, and many systems have been developed including, for example, press facsimile systems, high speed photo type setting systems, cathode ray tube (CRT) display systems, high speed photographic recording systems such as that employing holography, and photo mask printing systems for IC production. In the CRT display systems, a variety of fluorescent materials which emit light on the order of a microsecond are used. Each fluorescent material has at least one fluorescent energy peak in the spectrum; for example, P-2 fluorescent material has an energy peak at about 545 nm, P-4 at about 560 nm, P-22D at about 525 nm, P-31 at about 520 nm, and P-20 at about 560 nm, respectively.
The processing speed of photosensitive materials using a silver halide photographic emulsion is always increasing and now the period required for development and fixing has become as short as about 60 to 120 seconds. Such a short processing period has made it rather difficult for the sensitizing dye to be removed from the emulsion layer, thus causing a residual tint in the processed material due to residual dye. Such tinting is one of the factors deteriorating the quality of the final image.
Spectral sensitization in the green region is frequently accomplished using dyes selected from the merocyanine, hemicyanine and tri-nuclear cyanine dyes. However, all of these sensitizing dyes tend to give rise to too broad a spectral response, thus these dyes are inappropriate for sensitization in a narrow, particular range of the spectrum. In addition, the absolute degree of sensitivity is often insufficient; especially in the sensitization of a high-speed silver iodobromide photographic emulsion the degree of sensitization is unfortunately low. These sensitizing dyes also suffer from the lack of supersensitizers therefor. For the present purpose of sharp, narrow band sensitization, application of a J-aggregate cyanine dye is, as is already known, recommended. The following patents describe such techniques using dyes mentioned below, e.g., imidazolocarbocyanine dye as disclosed in U.S. Pat. Nos. 2,701,198, 2,945,763, 2,973,264, 3,173,791, 3,364,031, 3,397,060, 3,506,443, 3,617,294 and 3,663,210, Japanese Pat. application No. 4936/1968, and German (OLS) 2,011,879, and 2,030,326; imidaoxacarbocyanine dye as disclosed in Japanese Pat. application 14030/1969, and pseudo-isocyanine dye as disclosed in German (OLS) 1,936,262 and French Pat. 1,488,057.
Although some of these techniques can achieve a superior level of sensitivity, most of them tend to give too broad a spectral response and a residual tint above permissible levels, thus failing to be practiced industrially. A technique using styryl dyes as supersensitizers is known as described in, for example, British Pat. No. 498,031, German Pat. No. 1,051,116, U.S. Pat. Nos. 2,313,922, 2,316,268, 2,533,426 and 2,852,385. However, unfortunately only a very low sensitivity is obtained using this technique for an exposure on the order of a microsecond, i.e., a far briefer exposure time scale than the usual one. In other words, emulsions based on this technique suffer from a marked reciprocity law failure towards high intensity (brief) exposure.